They seem to be used as a metaphor/symbol for all sorts of things nowadays. I would just like to confirm that in the EU we don't have to use them as a means of indicating which areas are suffering punch-ups on garage forecourts because of lack of fuel. Nor are there noticeable shortages of anything else. Including attractive mixed tomatoes in Lidl, where I went to buy flour.
Sourdough update: I gave up on my pan de cristal, poured the sourdough soup into a roasting tin, and made focaccia. It may look convincing (extra) but the texture was really not focaccia-like. However it was edible and tasted OK with a glass of wine just now. S is back now after his lovely week's walking.